Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters, which treat accelerated rhythms of the heart such as fibrillation or implantable pacemakers, which maintain the heart rate within a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
Pacemakers deliver pacing pulses to the heart to cause the stimulated heart chamber to contract when the patient's own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves) and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Implantable cardioverter-defibrillators (ICD's), encapsulated in a conductive housing or enclosure, are generally implanted in the left pectoral region of a patient and electrically connected to the heart with one or more electrode carrying leads. One lead includes at least one set of electrodes positioned in the right ventricle. An arrhythmia detector detects ventricular arrhythmias, such as ventricular fibrillation. When such an arrhythmia is detected, a pulse generator delivers a defibrillation output pulse from the defibrillation electrode in the right ventricle to the conductive housing to terminate the arrhythmia.
Electrodes in the leads connected to the ICD may be positioned throughout the heart and attached to the tip of a lead or along the body of the lead. The electrodes are the components of the lead where the electrical current generated in the ICD is applied to the tissue of the patient.